Continuing 21st Street's two northbound lanes past Bott Avenue and up to the Fountain Creek bridge has been added to an upcoming interim safety project at 21st
and Highway 24.
Previously announced project improvements - based on an analysis of traffic accidents at that intersection - feature a northbound 21st Street right-turn-only lane,
elimination of the broadly curved merging lane for right turns onto eastbound Highway 24 and an offset of the highway's left-turn lanes to improve visibility.
These upgrades are now planned for “early to mid-2009,” according to Rob Kidder of City Engineering.
Funded chiefly by the Pikes Peak Rural Transportation Authority (RTA), the work would be apart from the proposed Westside Highway 24 expansion project by the
Colorado Department of Transportation (CDOT), which is tentatively scheduled for an interchange at that intersection no earlier than the year 2026.
Keeping an extra northbound lane on 21st Street will help move traffic through the highway intersection and thus avoid back-ups that can extend beyond Bott Avenue
and block business accesses along 21st, explained Colleen Dawson of Traffic Engineering.
The extra lane will have to be dropped north of the intersection, between Naegele Road and the Fountain Creek bridge, because the bridge is too narrow to allow
more than one lane each way. Merging at that point shouldn't be a problem, Dawson believes. “Because vehicles tend to accelerate at different rates, they can
merge… with relative ease,” she said. “This also helps from a signal timing aspect in that more vehicles can travel through the [highway] intersection on a shorter signal
phase, which allows more green time to be given to Highway 24.”
Dawson also outlined a change in the project's northbound 21st Street right-turn configuration onto eastbound Highway 24. Like the preliminary plan that had been
presented last year, the final plan also shows the elimination of the curving merge, which, according to police studies, has helped cause multiple rear-end collisions.
However, after closer examination of the geography, Traffic Engineering has determined there isn't room for the eastbound acceleration lane shown on the prelim plan.
The narrow bridge over Fountain Creek east of 21st is only 260 feet away, whereas modern safety standards require about 750 feet for that type of acceleration lane,
Dawson said. This means that right-turners on northbound 21st will have to turn directly into through traffic (allowable on green lights or on a red light if the driver
thinks it's safe). Asked how safe that will be, considering the high speeds sometimes on Highway 24, Dawson said such a design is not that unusual and exists on other
“major arterial” streets (such as Academy Boulevard).
One other difficulty for northbound-21st right-turners in the new configuration could result from how the road comes up to the highway. The angle is noticeably tighter
than 90 degrees. Typically, when the city builds right turns onto busy streets, a 90-degree corner is rounded to make the turn less abrupt. At 21st Street, because of
the tight angle to start with, the rounding will essentially just improve the right-turn-only lane to 90 degrees, a project drawing indicates.
Asked why more extensive improvements are not possible at the intersection, Dawson noted that the Colorado Department of Transportation (CDOT) plans to build
an interchange there in the future (currently estimated at 15 or 20 years), and this fact, along with a tight budget, plays into how much the city chooses to invest into a
city street intersection with a CDOT-maintained highway.
The upgrade plan was developed after the 2006 city “dangerous intersections” list showed 21st and Highway 24 as the second worst in the city. Rear-end accidents
were a particular problem, according to police statistics.
A nearby Westside intersection, Highway 24 and 26th Street, ranked ninth in the same 2006 list and was simultaneously analyzed. The safety problem is chiefly left
turns from the highway in either direction onto 26th, Dawson said. Improvements there had at one time been considered for construction around the same time as
those at 21st. However, Kidder said, because of “recent budget constraints,” funding for 26th won't be available now until 2011.